THE ORIGIN OF ANIMAL SPECIES 461 



which are permanent characteristics of forms lower than 

 themselves, is the basis of the Recapitulation Theory, 

 which holds that each animal bears the marks of its own 

 ancestry and reveals its parentage in its own develop- 

 ment. 



(4) Evidence from Paleontology. Although only a 

 very small part of the earth's crust has been examined, 

 the fossil animal remains already found furnish the pri- 

 mary and most direct evidence in favor of the theory of 

 descent with modification, and they show that the pro- 

 cess began as far back in geological history as they can 

 be traced. The conclusions reached from the study of 

 fossils may be stated in the words of the great paleon- 

 tologist Zittel : (i) All stratified sedimentary rocks 

 (with the exception of metamorphic rocks) inclose, more 

 or less richly, fossils, and thus prove that the earth, for 

 an immeasurable length of time before the appearance 

 of man, was inhabited by organisms. (2) Fossils of the 

 oldest and deepest strata represent extinct species, and 

 for the most part extinct genera; only in the more 

 recent strata are found forms which are identical with 

 those now living. The deeper down we penetrate in 

 the series of strata, the more divergent are the fossils 

 from the forms now living ; and, on the contrary, rising 

 from the earliest to the more recent formations there 

 is a continuously increasing resemblance to the present 

 creation. (3) The different fossil faunas and floras 

 follow each other the world over in the same regular 

 sequence ; the formations stratigraphically nearer to 

 each other contain the most similar fossils, and those 

 most separated in age present the greater differences. 

 (4) Constant change characterizes the evolution of the 

 organic creation. Species of one geological formation 

 are either completely or partly replaced by other species 

 in the next superimposed strata. (5) Each species, like 



